1. Field of the Invention
The present invention relates to a self-adjusting presser foot for assembler sewing machines, of the type comprising: a small bearing frame fixedly fastened to a presser bar of the sewing machine; a shoe which is engaged at the lower part thereof to the small bearing frame, is arranged to act in elastic-thrust relationship on two fabric pieces to be assembled, and exhibits a longitudinal opening designed to receive and guide two opposite edges of said fabric pieces disposed in mutual side by side relation, as well as a transverse slit for access to sewing means adapted to firmly joint said fabric pieces by a seam of stitchings along said opposite edges; cutting means acting close to said longitudinal opening for trimming the side by side fabric piece edges, upstream of the transverse slit; and guide means formed at one end of said longitudinal opening and acting downstrean of the cutting means and upstream of the transverse slit in order to mutually overlap the trimmed fabric piece edges.
2. Prior Art
It is known that assembler sewing machines are used to mutually joint two or more fabric pieces being part of a workpiece and disposed in coplanar side by side relation on the sewing machine worktable. In greater detail, the mutual assembling of the two fabric pieces takes place through mutual overlapping and sewing of the opposite edges thereof.
For the purpose, in assembler sewing machines a particular presser foot is normally provided which is conceived so that it can trim the opposite edges of the fabric pieces to be joined together and lead the trimmed edges under a condition of mutual overlapping to the area in which the sewing is to be carried out.
To this end, the presser foot is substantially comprised of a small bearing frame fixedly connected to the presser bar of the sewing machine and carrying a shoe at the lower part thereof which is designed to elastically act against the fabric pieces disposed in mutual side by side relation, over feed dogs that, as known, act through a needle bar associated with the machine worktable in order to cause the movement of the fabric pieces in the sewing direction.
The shoe has a longitudinal opening at the front into which the opposite edges of the fabric pieces turned upwardly and mutually mating are introduced.
Operating at the longitudinal opening is cutting means substantially comprising a fixed knife rigidly engaged to the shoe so that it projects sideways towards the inner part of the opening and a movable knife elastically urged against the fixed knife and operated with oscillatory motion in the transverse direction relative to the opening. In this way, the cooperation between the fixed and movable knives causes the trimming of the edges of the fabric pieces that, by effect of the workpiece moving forward, are gradually introduced into the longitudinal opening.
The longitudinal opening terminates in a shaped end so that the trimmed fabric piece ends can be disposed in the same plane as the fabric pieces in mutual overlapping relation before their having access to the area in which by means of one or more needles cooperating with one or more loopers, a seam of stitches fixedly joining said edges is formed.
Usually, in combination with the presser foot of the above described type a needle-plate is used which is provided with a stepped surface recess extending substantially along the sewing direction. In this way, one of the fabric pieces is disposed, at least close to its edge to be sewn to the other fabric piece, according to a plane slightly lower than the other fabric piece in order to avoid the thickness increase due to the overlapping of said edges causing a partial loss of contact between the shoe and the individual fabric pieces.
In order to compensate for thickness variations in the workpiece as much as possible, it is also provided for the shoe to act on the fabric pieces by a pair of elastic compensating foils substantially in the shape of small leaf springs and extending parallelly along the lower shoe surface.
However, notwithstanding the above described expedients, presser feet presently used in the above described assembler sewing machines still have some drawbacks. In fact, due to the fact that the pressure exercised by the shoe on the individual fabric pieces is caused not only by the elastic thrust exerted through the presser bar, but also by the extent of elastic flection undergone by the elastic foils, it is very difficult to achieve a uniform specific pressure throughout the whole contact surface between the shoe and the fabric pieces. In particular, also due to the surface recess present on the needle-plate, it often happens that there is a fabric piece having a specific pressure lower than that produced on the other fabric piece. Clearly, this situation can impair the correct feeding of the workpiece while a sewing is being carried out.
In addition and above all, it is very difficult to achieve a uniform working when, during the workpiece advancing, the shoe of the presser foot meets thickness increases due for example to the presence of folded back edge pieces and/or inserts of various kinds, such as tapes or the like, previously applied to the fabric pieces.
Under this situation, in fact, it often happens that the fabric piece edges located close to the thickness increase are not overlapped. On the contrary, in these areas the fabric piece edges slide away from each other and are undesirably spread apart before the sewing is carried out.
In addition, since the shoe is compelled to lift in order to step over the thickness increase, sewing irregularities in the areas adjacent to this thickness increase also occur.
It is also noted that, since the fixed knife is directly mounted on the shoe, the lifting of the latter also gives rise to irregularities in the trimming of the fabric piece edges.